The invention relates to a scanning head for eddy-current testing and to a method for producing such a scanning head. The invention furthermore relates to an eddy-current test method.
An article entitled xe2x80x9cNon-destructive Testing of Corrosion Effect on High-temperature Protective Coatingsxe2x80x9d by G. Dibelius, H. J. Krichel and U. Reimann, in VGB Kraftwerkstechnik 70 (1990), No. 9 describes an eddy-current test on gas turbine blades. Gas-turbine blades are subject to severe mechanical and thermal stresses. Testing such blades for material faults, such as cracks, is essential to operational safety. Gas-turbine blades are, as a rule, provided with a protective coating. The quality of that coating can be checked, inter alia, by using an eddy-current test method. In that case, an excitation coil is used to produce a magnetic alternating field which causes eddy currents in the material to be tested. The eddy currents in turn cause a magnetic alternating field, which is measured by using a detector coil. Material faults have a characteristic influence on the measured magnetic field, and can thus be detected.
A book entitled xe2x80x9cZerstxc3x6rungsfreie Werkstxc3xcck- und Werkstoffprxc3xcfungxe2x80x9d [Non-destructive Workpiece and Material Testing] by Siegfried Steeb, Expert-Verlag Bxc3x6blingen, 1988, mentions that one effect which has a negative influence on an eddy-current test is a so-called lifting-off effect. The lifting-off effect results from the detector coil lifting off or changing its distance from the test object.
It is accordingly an object of the invention to provide a scanning head for eddy-current testing, a method for producing a scanning head for an eddy-current test and an eddy-current test method, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type, which make it possible to carry out a quick eddy-current test and in which a lifting-off effect is low.
With the foregoing and other objects in view there is provided, in accordance with the invention, a scanning head for eddy-current testing of a test object having a test surface, comprising a film base having a film surface; a film disposed at the film surface; a probe coil configuration integrated into the film and having an excitation coil and a detector coil; and the film surface matched to the test surface or to a part of the test surface, permitting gap-free movement of the film over the test surface.
Moving the scanning head over the test surface thus results at most in a gap between the film and the test surface caused by manufacturing tolerances or locations of unevenness. The film essentially slides over the test surface.
Matching the film surface to the test surface allows the scanning head to be moved over the test surface virtually without lifting off at all, so that essentially no lifting-off effect occurs. The use of a probe coil configuration in or on a film also allows the scanning head to be constructed with a large probe area. In consequence, the test time can be kept short since a large area is tested whenever the scanning head is passed over the test surface.
The detector coil may be a single coil, but may also be formed from two coils, in particular two coils wound in mutually opposite senses (gradiometer configuration). The detector coil is then essentially sensitive only to magnetic field gradients by using such a gradiometer configuration. In particular, the excitation coil induces at most a small signal in the detector coil.
In accordance with another feature of the invention, the film base is preferably composed of a flexible material, at least adjacent the film surface. In consequence, the film surface can be matched even better to the test surface by pressing the film base against the test surface.
In accordance with a further feature of the invention, the excitation coil and the detector coil preferably have a mutual inductance of less than 1 nH, in particular less than 100 pH. This structure keeps crosstalk from the excitation coil to the detector coil at a low level.
In accordance with an added feature of the invention, the excitation coil preferably also has a conductor cross-section of more than 10xe2x88x923 mm2.
In accordance with an additional feature of the invention, the film is preferably at least partially provided with a cooling coating, which is thermally highly conductive and electrically poorly conductive. The film is preferably constructed to be thermally highly conductive and electrically poorly conductive. Furthermore, the film base is preferably constructed to be thermally highly conductive. These measures serve to keep heating low or to carry away the heat that is produced effectively and without damage. These measures make it possible, in particular, to pass a high excitation current through the excitation coil. A high excitation current is desirable in order to increase the measurement sensitivity. A high excitation current results in losses, and thus heating, as a result of the electrical resistance.
In accordance with yet another feature of the invention, the test surface has a roughness with a mean roughness length, and the probe coil configuration preferably has an extent which is considerably greater than the mean roughness length, in a direction lying in the film. In this way, lifting-off effects caused by the roughness are averaged out.
In accordance with yet a further feature of the invention, the detector coil preferably has a greater extent in the film surface in a longitudinal direction than in a transverse direction lying at right angles to the longitudinal direction. This refinement allows increased detector coil sensitivity for elongated material faults, such as cracks which are oriented in the longitudinal direction, when the detector coil is moved transversely with respect to the longitudinal direction.
Furthermore, the detector coil in the film surface can preferably be matched to an imaginary square-function envelope line in such a way that it touches all four sides of the envelope line. As a result of the symmetry of such a structure, the sensitivity of the detector coil is independent of the orientation of elongated material faults.
In accordance with yet an added feature of the invention, the probe coil configuration can preferably be read by a read unit which contains a SQUID sensor. The use of a SQUID sensor increases, in particular, the sensitivity and the signal-to-noise ratio of the measurement apparatus. The probe coil configuration in this case is part of a flux transformer for transmitting the magnetic field to be measured to the highly-sensitive SQUID sensor.
In accordance with yet an additional feature of the invention, the test surface is preferably formed by a wall of a groove in the test object, and the film surface is matched to the wall. As a rule, access to a groove with a conventional scanning head for eddy-current testing is difficult. Even a groove can be tested easily and without any significant interference from the lifting-off effect, by matching the film surface to the groove wall. An eddy-current test can thus be carried out easily and quickly even in the case of complex geometries, such as those formed by a groove.
In accordance with again another feature of the invention, the test surface is preferably a part of a surface of a turbine blade having a root part and a blade section leading edge, in particular a part of a surface of the root part or of a surface of the blade section leading edge. The configuration of the scanning head allows quick and efficient testing of the turbine blades for material faults. The root part and the blade section leading edge of a turbine blade are particularly subject to severe stresses and must be tested regularly. In this case, the adapted scanning head also allows testing to be carried out outside the laboratory, for example directly on the turbine. The capability of carrying out a quick eddy-current test in this case can keep expensive inspection time short. Both gas-turbine and steam-turbine blades as well as the blades of turbine propulsion systems can be tested.
In accordance with again a further feature of the invention, the probe coil configuration is preferably constructed as a photolithographically produced conductor configuration. Particularly in the case of a gradiometric probe coil configuration, that is to say in the case of a detector coil which is formed from two coils wound in opposite senses, the photolithographically produced probe coil configuration allows the two coils of the detector coil to be matched well.
With the objects of the invention in view, there is also provided a method for producing a scanning head for eddy-current testing of a test object having a test surface with a shape, which comprises attaching a moldable material to the test surface without gaps; forming a film base with the material for a probe coil configuration integrated in a film; and matching the film base to the shape of the test surface.
The advantages of such a method correspond to the statements relating to the advantages of the scanning head for an eddy-current test.
In accordance with another mode of the invention, the film is preferably in close contact with the test surface and the material is cast over the film and cured. Such casting of a film which is in close contact with the test surface results in a scanning head that is matched to the test surface being produced in a simple and quick manner.
With the objects of the invention in view, there is additionally provided an eddy-current test method for testing a test object having a test surface, which comprises placing a probe coil configuration integrated into a film at a film surface of a film base on a scanning head; matching the film surface to the test surface for moving the film over the test surface without gaps; and moving the scanning head over the test surface.
The advantages of such an eddy-current test method correspond to the statements relating to the advantages of a scanning head for an eddy-current test.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a scanning head for eddy-current testing, a method for producing a scanning head for an eddy-current test and an eddy-current test method, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.